msm: mdss: clear function pointers after releasing control path

Once control path is freed, the function pointers associated as part of
the control interface should be cleared to avoid them being called when
the control path is assigned to a different interface.

Change-Id: I635a00657341c06f3c52a4b264bd87b8bb20d783
Signed-off-by: Adrian Salido-Moreno <adrianm@codeaurora.org>